Preparation of films having good WVTR from highly filled polymers, usually polyolefins, are known. In the past a combination of a polyolefin, usually a polyethylene, with a filler, usually CaCO.sub.3, while very useful and widely used as a film with good WVTR, usually in combination with non-woven polymers (for use in diapers, adult incontinence devices, feminine hygiene articles, housewrap composites, roofing materials and the like), have had some limitations that were well known in the industry.
Among these limitations are a practical limitation in thickness (also expressed as basis weight) in that conventional Ziegler-Natta catalyzed polymers, more specifically linear low density polyethylene (LLDPE) highly filled film formulations could not generally be drawn down below 3 mils. The most obvious problem with such a limitation is that the user of the film could not make a product utilizing a lower thickness film, meaning that the cost of the film (usually sold on a weight basis) might have been higher than the application necessitated. A less obvious issue is that at lower thicknesses, for the same density resin at the same filler loading, the product would be relatively softer than higher thicknesses, an attribute of importance in any article that comes in contact with humans, such as apparel.
Another limitation of previous polyethylene/filler films is that for a given filler loading, with conventional Z-N catalyzed polyethylene resins, is WVTR, limited (on the upper end) by the amount of post-extrusion orientation that could be practically achieved. Additionally, the imperfections often found in conventional Z-N resins and films, such as gels, made reaching and maintaining a high rate of production difficult, and a high level of orientation might often lead to breaks, holes, or tear offs in the film leading to lower prime production rates.
Yet another limitation of the conventional Z-N filled and oriented films is related to both WVTR and production rates. Specifically, with a given conventional filled polyethylene, to attain a certain WVTR, a certain filler loading had to be used. In general, within limits, the higher the filler loading, the more difficult to process ( the above referenced production problems such as large void creation and tear offs are exacerbated by a higher filler loading, as the film maker seeks to maximize production rates).
U.S. Pat. No. 4,777,073 suggests a permeability and strength of polyethylene/filler combinations may be attained by combining a LLDPE described as being made using a Zeigler-Natta or chromium catalysts, with fillers such as CaCO.sub.3 present in the LLDPE from 15 to 35 percent by volume which is equivalent to 34-62% by weight.
There is a commercial need therefore for a polyethylene filler combination that will give a higher WVTR at a given filler loading, at an equivalent thickness. There is a similar need for a polyethylene filler combination that can deliver equivalent WVTR at lower filler loadings and can be made at a lower basis weight, than a conventional Z-N polyethylene/filler combination.